1. Field of the Invention
The present invention relates mainly to a high-speed, high-image quality full-color tandem image-forming apparatus and to a full-color tandem image-forming method for use in electrophotographic and electrostatic recording processes.
2. Description of the Related Art
Hitherto, there have been known a variety of image-forming apparatuses employing electrophotographic technology such as copying machines and printers.
Recently, use of full-color copying machines and printers and the like that employ color toners in yellow, magenta, and cyan as well as a black toner has been expanding rapidly.
An intermediate transfer belt is often used in these full-color copying machines and printers for improving the image-forming speed and the quality of images formed.
Full-color image-forming apparatuses employing such an intermediate transfer belt are equipped with multiple toner image-forming units corresponding respectively to four colors: yellow, magenta, cyan and black. Each toner image-forming unit includes a photoreceptor that rotates bearing an electrostatic latent image, and a developing device that develops the electrostatic latent image into a toner image on the surface of the photoreceptor by supplying a toner to the photoreceptor bearing the electrostatic latent images. In these image-forming apparatuses, image signals in four colors, yellow, magenta, cyan and black, are input respectively to the toner image-forming units corresponding to the respective colors, and in each toner image-forming unit, an electrostatic latent image is formed, based on the image signal, on the photoreceptor, which is previously electrostatically charged uniformly, and developed into a toner image. The four toner image-forming units are placed tandem in the circulating direction of the intermediate transfer belt, and the toner images formed in each toner image-forming unit are transferred and superimposed one by one on the circulating intermediate transfer belt (hereinafter, the transfer is referred to as the primary transfer). Subsequently, the superimposed toner image on the intermediate transfer belt is transferred (secondary transfer) and fused on a recording medium, to give the image on the recording medium.
On the photoreceptor after each primary transfer, there remain discharge products generated by electrostatic charge, toner particles not transferred and remaining on the photoreceptor, and the like. Blade cleaning method, wherein a blade, for example, made of a polyurethane rubber is brought into contact with the photoreceptor to scrape off the deposits, has been hither to practiced for removal of these discharge products, the residual toners after transfer, and the like. The blade cleaning method uses the residual toners after transfer, which are supposed to be removed, as the lubricant and abrasive for attaining favorable cleaning characteristics. Accordingly, presence of a smaller amount of residual toners after transfer, which is favorable in itself, causes disadvantages that it leads to decrease in lubricity and hence to the wear and crack of blade, or to decrease in polishing ability resulting in insufficient removal of the discharge products. Further, it also results in fluctuation in the amount of residual toner depending on the images formed in the full-color image-forming apparatuses that uses an intermediate transfer belt. For example, a single image is often formed or printed in great number in these high-speed high-throughput image-forming apparatuses. In such a case, the toner image is often not formed on some of the electrostatic latent image-bearing bodies, depending on the kind of the image (e.g., business card, flier in a particular color or in black and white, or the like), sometimes leading to significant deficiency in the amount of residual toners after transfer. There are image-forming devices which, when the output number reaches a certain value or the ratio of the image density to the cumulative rotation number of the photoreceptor reaches a certain threshold value, suspend forming the image once and then form a toner band on the photoreceptor by supplying the toner to the photoreceptor independent of the image to be formed, and thus prevent the decrease in lubricity and polishing ability.
However, as suspension of image forming leads practically to decrease in image-forming efficiency and the supply of the toner independent of the formed image onto the photoreceptor increases the amount of the toner consumed, such image-forming apparatuses do not allow formation of the toner band sufficient for allowing advantageous effects, but rather lead to wear and crack of the blade therein and deficiency in cleaning ability and thus to defects in the image quality of formed images.
Alternatively, a method of applying a metal soap such as ZnSt or the like as the solid lubricant for the surface of photoreceptor and thus preventing increase in the friction between the surface of the-photoreceptor and the blade was proposed (e.g., see Japanese Patent Application Publication No. 51-22380). In the method, the amount of the metal soap applied onto the photoreceptor is determined in advance in such a manner that the lubricity between the surface of the photoreceptor and the blade is kept at a suitable level.
However, even in such toner image-forming units having the photoreceptor, the surface thereof being coated with the metal soap, increase in the amount of residual toners after transfer leads to increase in the amount of the fatty acid metal salt scraped off, as the residual toners after transfer are used as the abrasive. In image-forming apparatuses wherein multiple toner image-forming units are placed side by side in the circulating direction of an intermediate transfer belt, occurs occasionally a phenomenon called “retransfer” that part of the toner image previously transferred onto the intermediate transfer belt migrate onto the surface of the photoreceptor whereon the next toner image is borne. Electrostatic latent image-bearing bodies placed further downstream in the circulating direction of the intermediate transfer belt absorb a greater amount of toners by the retransfer. The toners adhered by the retransfer also function as the abrasive together with the residual toners after transfer, and accordingly a greater amount of the fatty acid metal salt on the photoreceptor located further downstream is scraped off, resulting in deterioration of the lubricity between the surface of the photoreceptor and the blade. Increase in the amount of fatty acid metal salt applied onto the electrostatic latent image-bearing bodies, which are located further downstream, is suggested (e.g., see Japanese Patent Application Laid-open No. 2001-34111), but adoption of such a method does not allow the image-forming apparatuses to cope with the fluctuation in the amount of residual toners dependent on formed images or to keep the favorable cleaning characteristics for an extended period of time.